Method of Thermal Management of Fuel Cell System Using Mea Temperature Estimation

This application claims, under 35 U.S.C. § 119(a), the benefit of priority from Korean Patent Application No. 10-2024-0092781, filed on Jul. 15, 2024, the entire contents of which are incorporated herein by reference.

The present disclosure relates to a method of thermal management of a fuel cell system, and more particularly to a method of thermal management of a fuel cell system based on the temperature of a membrane electrode assembly (MEA).

A fuel cell is a battery that generates electrical energy by electrochemical reaction between a fuel and an oxidant. The unit cells of such a fuel cell are connected in series to form a fuel cell stack for high output.

Generally, in a fuel cell stack, heat generation and temperature rise occur along with an electrochemical reaction of each unit cell (i.e., unit fuel cell). The unit fuel cell is made of a thin membrane structure, and the higher the frequency of exposure to high temperatures, the greater the likelihood of holes occur. This may cause coolant leakage from the fuel cell stack and may result in a situation in which power output of the fuel cell stack is difficult.

Accordingly, in order to protect the fuel cell stack, a fuel cell system mounted to a vehicle is configured to set the maximum allowable temperature of a membrane electrode assembly (MEA), which is the reactor of the fuel cell stack, to a predetermined temperature and to perform thermal management through circulation of coolant (i.e., stack coolant) supplied to the fuel cell stack so that the fuel cell stack is prevented from operating at a temperature higher than or equal to the predetermined temperature.

As such, low-temperature stack coolant is fed into the fuel cell stack and receives heat generated from the fuel cell stack, increasing the temperature thereof. Thereafter, the stack coolant is discharged from the fuel cell stack, is fed into a radiator, is cooled through the radiator, and then circulates back to the fuel cell stack. Thermal management of the fuel cell stack is achieved by circulation of the stack coolant.

A conventional fuel cell system is configured to perform thermal management of the fuel cell stack based on the outlet temperature of the stack coolant discharged from the fuel cell stack, not the temperature of the membrane electrode assembly. The outlet temperature of the stack coolant may be measured at the coolant outlet of the fuel cell stack, and may also be referred to as a coolant outlet temperature.

In order to directly measure the temperature of the membrane electrode assembly, it is necessary to insert and mount a temperature sensor between unit fuel cells.

The fuel cell stack is assembled and configured with a fastening force that may optimize performance of the fuel cell stack by minimizing contact resistance between unit fuel cells, and the surface pressure and cell pitch (i.e., distance between unit fuel cells) of the fuel cell stack are determined depending on the fastening force.

In cases in which a temperature sensor is installed between unit fuel cells to measure the temperature of the membrane electrode assembly, the surface pressure and cell pitch of the fuel cell stack are affected thereby, which may ultimately cause performance degradation of the fuel cell stack.

Accordingly, in the conventional fuel cell system, thermal management of the fuel cell stack is performed based on the coolant outlet temperature raised by heat generated from the fuel cell stack, rather than measuring the temperature of the membrane electrode assembly.

More specifically, the conventional fuel cell system is configured to recognize the coolant outlet temperature when the membrane electrode assembly reaches a predetermined maximum allowable temperature as the maximum allowable outlet temperature of the stack coolant and to perform thermal management of the fuel cell stack based on the maximum allowable outlet temperature of the stack coolant.

However, as the unit fuel cell deteriorates, the heating value generated when producing electrical energy increases, so even if the coolant outlet temperature of the fuel cell stack is the same, the temperature of the membrane electrode assembly may differ depending on the extent of deterioration of the unit fuel cell. In addition, the operable range (i.e., output current range) of the fuel cell stack may vary depending on the temperature change of the membrane electrode assembly. Therefore, the conventional fuel cell system cannot reflect changes in the operable range due to deterioration of the fuel cell stack.

Moreover, the conventional fuel cell system is configured to select the maximum flow rate of the coolant pump for the fuel cell stack based on the maximum allowable outlet temperature of the stack coolant. However, during actual operation of the fuel cell system, when the temperature of the stack coolant is the maximum allowable outlet temperature, the temperature of the membrane electrode assembly is lower than the maximum allowable temperature of the membrane electrode assembly, so the maximum flow rate of the coolant pump may be overdesigned.

The present disclosure has been made keeping in mind the problems encountered in the related art, and an object of the present disclosure is to provide a method of thermal management of a fuel cell system capable of estimating the temperature of a membrane electrode assembly (MEA) of a fuel cell stack based on data that may be collected in real time and performing thermal management of the fuel cell stack based on the estimated temperature of the MEA.

The objects of the present disclosure are not limited to the foregoing, and other objects of the present disclosure not mentioned herein will be clearly understood by those skilled in the art from the following description.

In order to accomplish the above object, the present disclosure provides a method of thermal management of a fuel cell system, including determining, by a controller, a heating value and thermal resistance of a fuel cell stack during driving of a vehicle, estimating a temperature of a membrane electrode assembly (MEA) provided to the fuel cell stack based on the heating value and the thermal resistance of the fuel cell stack, and performing thermal management of the fuel cell stack based on the estimated temperature of the MEA.

According to an embodiment of the present disclosure, data for estimating the temperature of the MEA may include an output current and an output voltage of the fuel cell stack, a coolant inlet temperature and a coolant outlet temperature of the fuel cell stack, and a coolant flow rate supplied to each unit cell of the fuel cell stack.

Also, the coolant flow rate of the unit cell may be determined based on a rotation speed of a coolant pump configured to deliver coolant to the fuel cell stack and an opening rate of a coolant control valve configured to control the coolant flow rate supplied to the coolant pump.

Also, according to an embodiment of the present disclosure, the controller may be configured to determine the heating value of the fuel cell stack based on the output current and the output voltage of the fuel cell stack and the number of unit cells constituting the fuel cell stack.

Also, according to an embodiment of the present disclosure, the controller may be configured to estimate the thermal resistance of the fuel cell stack based on the coolant inlet temperature and the coolant outlet temperature of the fuel cell stack and the coolant flow rate per unit cell.

Also, according to an embodiment of the present disclosure, the controller may be configured to estimate the thermal resistance of the fuel cell stack using a stack thermal resistance determination model obtained by nonlinear regression analysis, and the stack thermal resistance determination model is configured to determine the thermal resistance of the fuel cell stack based on the coolant inlet temperature, the coolant outlet temperature, and the coolant flow rate.

Also, according to an embodiment of the present disclosure, the controller may be configured to estimate the temperature of the MEA based on the heating value and the thermal resistance of the fuel cell stack and the coolant outlet temperature.

Also, according to an embodiment of the present disclosure, in performing the thermal management of the fuel cell stack, a maximum allowable output current of the fuel cell stack, a target rotation speed of the coolant pump, and a target opening rate of the coolant control valve may be determined based on the estimated temperature of the MEA.

Also, according to an embodiment of the present disclosure, the controller may be configured to determine a maximum allowable output current of the fuel cell stack based on the estimated temperature of the MEA and to limit the output current of the fuel cell stack to less than or equal to the maximum allowable output current.

Also, according to an embodiment of the present disclosure, in performing the thermal management of the fuel cell stack, operation of the coolant pump and the coolant control valve may be controlled based on the target rotation speed and the target opening rate.

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Matters included in the accompanying drawings are schematically illustrated to easily describe embodiments of the present disclosure and may be different from actual forms.

The present disclosure relates to a method of thermal management of a fuel cell system capable of estimating the temperature of a membrane electrode assembly (MEA) provided to each unit cell (i.e., unit fuel cell) of a fuel cell stack and performing thermal management of the fuel cell stack based on the estimated temperature of the MEA.

In the present disclosure, the temperature of the MEA is estimated and determined based on actual measurement data that may be collected in real time from the fuel cell system without using a separate temperature sensor configured to measure the temperature of the MEA.

In the present disclosure, since the temperature sensor configured to measure the temperature of the MEA is not provided between unit fuel cells, it is possible to fundamentally prevent performance degradation of the fuel cell stack caused by directly providing the temperature sensor between unit fuel cells.

1 FIG. shows part of a thermal management system using coolant among the components of a fuel cell system mounted to a vehicle.

100 100 100 In order to protect the fuel cell stackfrom deterioration, the fuel cell system is configured to perform thermal management so that the fuel cell stackdoes not operate at a temperature higher than or equal to a predetermined temperature. To this end, the fuel cell system may include a thermal management system configured to manage heat generation of the fuel cell stack.

200 200 100 The fuel cell system includes a fuel cell controllerconfigured to perform overall control of the system. The fuel cell controllermay serve to control thermal management of the fuel cell stack.

2 FIG. 3 FIG. shows a simplified structure of a fuel cell stack, andshows a simplified structure of a unit fuel cell.

2 FIG. 3 FIG. 100 110 110 111 113 111 112 111 113 As shown in, the fuel cell stackincludes a plurality of unit fuel cellsstacked and arranged in a row. As shown in, each unit fuel cellis a unit cell that generates electrical energy and includes an MEA, a separator platestacked on each of both sides of the MEA, and a gas diffusion layerdisposed between the MEAand the separator plate.

111 The MEAis configured to generate electrical energy by inducing chemical reaction between a fuel and an oxidant, and, although not specifically shown, includes an electrolyte membrane with selective ion permeability and a cathode and an anode disposed on respective sides of the electrolyte membrane.

113 111 112 111 113 The separator platehas passages for the flow of fuel and air and serves to supply fuel and air to the MEAthrough the passages. The gas diffusion layerfacilitates diffusion of gas between the MEAand the separator plate.

113 113 116 115 111 114 111 4 FIG. 4 FIG. In addition, a coolant passage for the flow of coolant is provided at the outer side of the separator plate. Referring to, the separator plateis provided with a fuel passageand an air passagefor the flow of fuel and air at the inner side facing the MEA, and also with a coolant passageat the outer side corresponding to the opposite side of the MEA.shows part of the cross-sectional structure of the fuel cell stack.

1 FIG. 100 117 118 100 117 110 114 100 118 Referring to, the fuel cell stackincludes a coolant inletthrough which coolant is fed and a coolant outletthrough which coolant is discharged. The coolant fed into the fuel cell stackthrough the coolant inletflows between the unit fuel cellsthrough the coolant passageand is discharged from the fuel cell stackthrough the coolant outlet.

100 118 111 118 111 118 117 Accordingly, the temperature of the coolant fed into the fuel cell stackincreases as it approaches the coolant outlet, and the temperature of the MEAalso increases as it approaches the coolant outlet. Briefly, the temperature of the MEAis higher in the region proximate to the coolant outletthan in the region proximate to the coolant inlet.

111 118 111 111 In the present disclosure, the temperature of the MEAis the temperature of the region closest to the coolant outletamong the entire region of the MEA, and may indicate the highest temperature among the temperatures of regions of the MEA.

1 FIG. 210 100 117 220 100 118 210 117 220 118 210 220 200 As shown in, an inlet temperature sensoris configured to measure the temperature of the coolant supplied into the fuel cell stackthrough the coolant inletand an outlet temperature sensoris configured to measure the temperature of the coolant discharged from the fuel cell stackthrough the coolant outlet. More specifically, the inlet temperature sensormay serve to measure the coolant temperature at the coolant inletand the outlet temperature sensormay serve to measure the coolant temperature at the coolant outlet. The coolant temperature values measured by the inlet temperature sensorand the outlet temperature sensormay be transmitted to the fuel cell controller.

100 117 100 230 230 100 100 260 260 230 240 230 240 Coolant for cooling the fuel cell stackis supplied to the coolant inletof the fuel cell stackthrough a coolant pump. The coolant pumped from the coolant pumpserves to cool the fuel cell stackwhile passing through the fuel cell stack, followed by flowing to a stack radiator. The coolant cooled while passing through the stack radiatoris supplied back to the coolant pumpthrough a coolant control valve. The flow rate of the coolant supplied to the coolant pumpis determined depending on the opening rate (%) of the coolant control valve.

200 230 240 100 100 230 240 240 230 250 230 250 230 240 100 The fuel cell controllermay be configured to control operation current (i.e., output current) of the coolant pump, the coolant control valve, and the fuel cell stackin order to perform thermal management due to heat generated from the fuel cell stackduring driving of a vehicle. The rotation speed of the coolant pumpmay be controlled and the opening rate (%) of the coolant control valvemay be controlled. Depending on the opening rate of the coolant control valve, the coolant flow rate supplied toward the coolant pumpvaries, and the coolant flow rate flowing from a coolant reservoirtoward the coolant pumpalso varies. The coolant reservoirmay be configured to store coolant and may serve to supply coolant to the inlet of the coolant pumpdepending on the opening rate of the coolant control valve. The output current of the fuel cell stackmay be limited to a predetermined current value.

117 100 117 100 117 118 100 118 100 118 In the present disclosure, the coolant temperature measured at the coolant inletof the fuel cell stackmay be referred to as a coolant inlet temperature. This is because the coolant temperature measured at the coolant inletof the fuel cell stackis similar or identical to the temperature of the coolant inlet. Also, the coolant temperature measured at the coolant outletof the fuel cell stackmay be referred to as a coolant outlet temperature. This is because the coolant temperature measured at the coolant outletof the fuel cell stackis similar or identical to the temperature of the coolant outlet.

200 111 200 111 100 230 240 Meanwhile, the fuel cell controllermay be configured to estimate the temperature of the MEAbased on data that may be collected and obtained from a sensor. The fuel cell controllerserves to estimate the temperature of the MEAusing the output current and output voltage of the fuel cell stackin addition to the coolant inlet temperature, the coolant outlet temperature, the rotation speed of the coolant pump, and the opening rate of the coolant control valve.

200 100 111 200 230 240 100 100 240 200 Also, the fuel cell controllermay be configured to perform control to appropriately maintain the temperature of the fuel cell stackbased on the estimated temperature value of the MEA. For example, the fuel cell controllermay serve to control the rotation speed of the coolant pumpand the opening rate of the coolant control valveto the maximum and to limit the output of the fuel cell stackby limiting the current that may be generated and output from the fuel cell stack. The coolant control valvemay be an electronic valve, the opening rate of which is controlled by driving an electric motor. Driving of the electric motor may be controlled by the fuel cell controller.

5 FIG.A 5 FIG.B is a flowchart showing a thermal management process depending on the output and heat generation of the fuel cell stack during driving of a vehicle, andis a flowchart showing a thermal management process of the fuel cell system based on the MEA temperature.

200 111 The fuel cell controlleris able to estimate the temperature of the MEAall the time under conditions in which the fuel cell system is operating.

5 FIG.B 111 200 51 100 200 52 200 111 53 100 111 54 55 Referring to, data necessary for estimating the temperature of the MEAduring driving of a vehicle is collected and obtained in real time by the fuel cell controller(S). The thermal resistance and heating value of the fuel cell stackare calculated and determined based on the collected data by the fuel cell controller(S). By the fuel cell controller, the temperature of the MEAis estimated and determined based on the determined thermal resistance and heating value (S) and thermal management of the fuel cell stackis controlled based on the estimated temperature of the MEA(S, S).

100 100 100 During driving of the vehicle, hydrogen and air are supplied to the fuel cell stackdepending on the required output of the fuel cell system (i.e., required output by a driver), and as power is generated and output by the fuel cell stack, heat is generated from the fuel cell stack.

5 FIG.A 200 100 10 100 20 100 30 40 100 111 200 50 Referring to, when the vehicle is driven by a driver depressing the accelerator pedal, by the fuel cell controller, the required output current of the fuel cell stackis determined based on the amount of pressure on the accelerator pedal (S) and the currently available output current of the fuel cell stackis determined (S). As such, by the fuel cell stack, current according to the driver's request is output and simultaneously heat is generated due to current generation (S, S), and thermal management of the fuel cell stackis performed based on the temperature of the MEAby the fuel cell controller(S).

5 FIG.B 54 55 200 111 100 100 230 100 240 230 200 230 240 100 Referring to, in Sand S, the fuel cell controllerserves to determine and control, based on the estimated temperature of the MEAin order to manage heat of the fuel cell stack, the maximum allowable output current of the fuel cell stack, the target rotation speed of the coolant pumpconfigured to deliver coolant to the fuel cell stack, and the target opening rate of the coolant control valveconfigured to control the coolant flow rate supplied to the coolant pump. The fuel cell controllerserves to control the operation of the coolant pump, the coolant control valve, and the fuel cell stackbased on the determined target rotation speed, target opening rate, and maximum allowable output current.

200 230 111 240 111 100 111 To this end, the fuel cell controllermay include a memory in which a rotation speed determination map, an opening rate determination map, and a maximum allowable output current determination map are stored. The rotation speed determination map is configured to determine the target rotation speed of the coolant pumpbased on the temperature of the MEA, the opening rate determination map is configured to determine the target opening rate of the coolant control valvebased on the temperature of the MEA, and the maximum allowable output current determination map is configured to determine the maximum allowable output current of the fuel cell stackbased on the temperature of the MEA.

100 200 10 100 200 20 100 200 100 100 30 60 100 5 FIG.A Also, the required output current of the fuel cell stackis determined by the fuel cell controllerbased on the amount of pressure on the accelerator pedal during driving of a vehicle (S). Referring to, the available output current of the fuel cell stackis determined by the fuel cell controllerbased on the operating state of the fuel cell system (S). Next, the fuel cell stackis driven and powered by the fuel cell controllerbased on the required output current of the fuel cell stack. As such, an output current less than or equal to the maximum allowable output current and the available output current is generated by the fuel cell stack(S) and the vehicle is driven based on the amount of pressure on the accelerator pedal (S). The required output current of the fuel cell stackincreases or decreases in proportion to the amount of pressure on the accelerator pedal.

1 FIG. 100 117 120 119 100 100 119 100 120 100 Referring to, the fuel cell stackincludes, in addition to the coolant inlet, a fuel inletconfigured to feed fuel and an air inletconfigured to feed air. In proportion to the required output current of the fuel cell stack, the amount of air supplied to the fuel cell stackthrough the air inletand the amount of hydrogen supplied to the fuel cell stackthrough the fuel inletare determined. Accordingly, the current and power output from the fuel cell stackmay be determined and controlled based on the amount of pressure on the accelerator pedal.

200 100 The fuel cell controllermay serve to check and monitor the amount of pressure on the accelerator pedal in real time and to determine the required output current of the fuel cell stackin real time based on the amount of pressure on the accelerator pedal.

100 200 100 100 100 100 As the accelerator pedal is depressed harder by the driver, the required output current of the fuel cell stackmay increase. Accordingly, the fuel cell controllerserves to control operation of the fuel cell stackso that the output current of the fuel cell stackincreases according to the driver's request. As such, the heating value of the fuel cell stackalso increases with an increase in the output current of the fuel cell stack.

111 111 200 51 111 100 200 In order to estimate the temperature of the MEA, data for estimating the temperature of the MEAis collected and obtained by the fuel cell controller(S). Specifically, data necessary to estimate the temperature of the MEA, which rises with an increase in the heating value of the fuel cell stack, is collected and obtained by the fuel cell controller.

200 230 240 100 More specifically, by the fuel cell controller, data such as the coolant inlet temperature value, the coolant outlet temperature value, the rotation speed (i.e., rate of rotation) of the coolant pump, the opening rate of the coolant control valve, and the output current value and output voltage value of the fuel cell stackis collected.

200 210 220 200 230 240 230 240 100 100 100 200 By the fuel cell controller, the coolant inlet temperature value and the coolant outlet temperature value may be obtained from the inlet temperature sensorand the outlet temperature sensor. Also, although not specifically shown, by the fuel cell controller, data for the rotation speed of the coolant pumpand the opening rate of the coolant control valvemay be collected using a sensor configured to detect the rotation speed of the coolant pumpand a sensor configured to detect the opening rate of the coolant control valve. Furthermore, the fuel cell system may include a current sensor configured to detect the output current of the fuel cell stackand a voltage sensor configured to detect the output voltage of the fuel cell stack. The output current value and the output voltage value of the fuel cell stackmay be transmitted to the fuel cell controllerfrom the current sensor and the voltage sensor.

200 230 240 230 240 110 200 The fuel cell controllermay serve to estimate the coolant flow rate based on the rotation speed of the coolant pumpand the opening rate of the coolant control valve. To this end, a coolant flow rate determination model may be pre-configured. The coolant flow rate determination model is configured to determine the coolant flow rate based on the rotation speed of the coolant pumpand the opening rate of the coolant control valve. The coolant flow rate determination model may serve to determine the coolant flow rate value based on a single unit fuel celland may be pre-configured and stored in the memory of the fuel cell controller.

200 100 210 220 52 By the fuel cell controller, the thermal resistance value of the fuel cell stackmay be estimated using the coolant inlet temperature and the coolant outlet temperature detected through the temperature sensors,and using the coolant flow rate determined through the coolant flow rate determination model (S).

3 FIG. 110 111 112 113 113 111 112 113 x As shown in, the unit fuel cellis configured such that the MEA, the gas diffusion layer, which is a porous medium, and the solid separator plateare stacked. The coolant flows at the outer surface of the separator plate. Accordingly, heat (q) generated by electrochemical reaction in the MEAis released into the coolant through the gas diffusion layerand the separator plate.

112 113 111 112 113 x As such, heat conduction occurs through the gas diffusion layerand the separator plateand heat convection occurs through the coolant. Accordingly, the temperature depending on the heating value (q) of the MEAdecreases in the order of the gas diffusion layer, the separator plate, and the coolant.

111 112 113 1 2 3 4 1 2 3 4 Here, when the temperature of the cathode side of the MEAis T; the temperature of the outer side of the gas diffusion layeris T; the temperature of the outer side of the separator plateis T; and the temperature of the coolant is T, the temperature decreases in the following order: T->T->T->T.

6 FIG. 3 FIG. 110 112 113 111 112 113 1 1 1 2 2 2 3 3 shows an equivalent thermal circuit in which thermal resistance depending on the stacked structure of the unit fuel cellshown inis represented as the thermal resistance value of each medium. Typically, a temperature difference occurs depending on the thermal resistance characteristics of each medium located on the path through which heat is emitted. Depending on thermal resistance of the gas diffusion layer(R=L)/(k×A)), thermal resistance of the separator plate(R=L/(k×A)), and thermal resistance of the coolant (R=1/(h×A)), the temperature gradually decreases in the order of the MEA, the gas diffusion layer, the separator plate, and the coolant.

As is well known, thermal resistance is a resistance value that represents a temperature difference between media through which heat of a heating element is transferred, and is an inherent characteristic depending on the shape factor and properties of the heat transfer media.

x 1 2 3 111 112 113 Equation 1 below represents the relationship among the heating value (q) of the MEA, thermal resistance (R) of the gas diffusion layer, thermal resistance (R) of the separator plate, and thermal resistance (R) of the coolant.

Equation 1 may be summarized as Equation 2 below.

2 110 112 113 tot tot tot 1 2 3 tot Here, U is the total heat transfer coefficient, A is the reaction area (m) of the unit fuel cell, and Ris the total thermal resistance (R). The total thermal resistance (R) is the sum of thermal resistance (R) of the gas diffusion layer, thermal resistance (R) of the separator plate, and thermal resistance (R) of the coolant. The total thermal resistance (R) is given in Equation 3 below.

1 2 1 2 3 112 113 112 113 2 Here, Lis the thickness (m) of the gas diffusion layer, Lis the thickness (m) of the separator plate, kis the thermal conductivity (W/mK) of the gas diffusion layer, kis the thermal conductivity of the separator plate(W/mK), and his the convective heat transfer coefficient of the coolant (W/mK).

1 2 3 1 2 3 100 112 113 The thermal conductivities (k, k) and heat transfer coefficient (h) are factors affected by the operating conditions of the fuel cell stack. Specifically, the thermal conductivities (k, k) of the gas diffusion layerand the separator plateare characteristic factors affected by the temperature of the coolant, and the convective heat transfer coefficient (h) of the coolant is a characteristic factor affected by the temperature and flow rate of the coolant.

111 112 113 110 In addition, it is assumed that the reaction areas (A) of the MEA, the gas diffusion layer, and the separator platebased on the unit fuel cellare all the same.

x 1 4 tot 1 111 111 111 111 As shown in Equation 2, the relationship among the heating value (q) of the MEA, the temperature (T) of the MEA, and the coolant temperature (T) may be represented as a relational expression of the total thermal resistance (R). As such, the temperature (T) of the MEAis the temperature of the electrode corresponding to the reaction side of the MEA, and particularly, the electrode is a cathode.

tot 1 4 x 100 111 111 Meanwhile, when the temperature of the MEA is actually measured, the temperature of the MEA shows nonlinear characteristics depending on the temperature and flow rate of the coolant and the current density of the fuel cell stack. Referring to Equation 2, the thermal resistance (R) of the fuel cell stackmay vary depending on the difference between the temperature of the MEAand the coolant outlet temperature (T-T) and on the heating value (q) of the MEA.

Specifically, the difference between the temperature of the coolant and the temperature of the MEA due to heat generation of the fuel cell stack is not fixed to a predetermined constant value but may vary depending on the condition of the coolant.

200 111 100 Accordingly, the fuel cell controllerserves to estimate the temperature of the MEAusing the current and voltage of the fuel cell stackand the temperature and flow rate of the coolant.

111 100 100 200 52 To estimate the temperature of the MEA, the thermal resistance of the fuel cell stackis estimated and the heating value of the fuel cell stackis determined by the fuel cell controller(S).

200 100 100 The fuel cell controllerserves to estimate the thermal resistance of the fuel cell stackusing a stack thermal resistance determination model obtained and configured by nonlinear regression analysis in order to apply and consider the influence of factors on the thermal resistance of the fuel cell stackand the influence of interaction between the factors.

100 110 100 100 100 100 100 The factors such as the coolant inlet temperature and the coolant outlet temperature of the fuel cell stackand the coolant flow rate of the unit fuel cellhave nonlinear characteristics with the thermal resistance of the fuel cell stack. Therefore, in order to ensure that the influence of the factors on the thermal resistance of the fuel cell stackand the interaction between the factors are included in the thermal resistance value of the fuel cell stack, the stack thermal resistance determination model is pre-configured by performing nonlinear regression analysis, and the thermal resistance of the fuel cell stackis determined using the stack thermal resistance determination model. The stack thermal resistance determination model is configured to determine the thermal resistance of the fuel cell stackdepending on the coolant inlet temperature, the coolant outlet temperature, and the coolant flow rate.

200 100 The stack thermal resistance determination model, which is a nonlinear regression model, may be stored in the memory of the fuel cell controllerand may serve to determine the thermal resistance of the fuel cell stackbased on the coolant inlet temperature, the coolant outlet temperature, and the coolant flow rate. Here, for the coolant flow rate, a coolant flow rate value determined through the coolant flow rate determination model may be used.

200 100 100 52 200 100 110 110 100 100 Also, by the fuel cell controller, the heating value (Qstack) of the fuel cell stackmay be calculated using the current and voltage values of the fuel cell stack(S). More specifically, by the fuel cell controller, the heating value (Qstack) of the fuel cell stackmay be determined based on the low theoretical reversible voltage (V_LHV) and cell voltage (V_cell) of the unit fuel cell, stack current (I), and the number (N_cell) of unit fuel cellsconstituting the fuel cell stack. The heating value (Qstack) of the fuel cell stackmay be calculated using Equation 4 below.

100 100 110 100 110 100 110 100 110 In Equation 4, the low theoretical reversible voltage (V_LHV) is a value determined as a reversible voltage that may be generated based on the low heating value (LHV) when reacting hydrogen and oxygen in the fuel cell stack, and may be set to, for example, 1.25 V. Also, in Equation 4, the stack current (I) is the output current of the fuel cell stack, and the cell voltage (V_cell) is the output voltage of the unit fuel cellsconstituting the fuel cell stack. It is assumed that the output voltages of the unit fuel cellsare all the same, and the output voltage of the fuel cell stackis determined by multiplying the output voltage of the unit fuel cellsconstituting the fuel cell stackby the number (N_cell) of the unit fuel cells.

200 100 Although not specifically shown, the fuel cell controllermay include a heating value calculator configured to calculate the heating value (Qstack) of the fuel cell stack.

200 111 100 53 111 By the fuel cell controller, the temperature (T_MEA) of the MEAmay be estimated and determined based on the thermal resistance (Rtot) and the heating value (Qstack) of the fuel cell stackand the coolant outlet temperature (Tout) (S). The temperature (T_MEA) of the MEAmay be determined using Equation 5 below.

200 111 53 230 240 111 54 200 111 55 By the fuel cell controller, the temperature of the MEAis estimated in Sand then the target rotation speed of the coolant pumpand the target opening rate of the coolant control valveare determined based on the estimated temperature of the MEAin S. Also, by the fuel cell controller, the maximum allowable output current of the fuel cell stack is determined based on the estimated temperature of the MEAin S.

200 230 240 230 240 100 100 By the fuel cell controller, the operation of the coolant pumpand the coolant control valveis controlled based on the target rotation speed and the target opening rate. As such, the coolant pumpis driven so that the rotation speed thereof reaches the target rotation speed, and the coolant control valveis driven so that the opening rate thereof reaches the target opening rate. The target rotation speed, the target opening rate, and the maximum allowable output current are determined as values that do not cause performance degradation due to deterioration of the fuel cell stackby maintaining the operating temperature of the fuel cell stackwithin a predetermined range.

200 100 Also, by the fuel cell controller, the output current of the fuel cell stackis determined based on the amount of pressure on the accelerator pedal, and is determined and limited to a value less than or equal to the maximum allowable output current.

100 100 111 111 111 111 100 100 As such, the maximum allowable output current of the fuel cell stackmay be determined as a maximum output current value that may be generated in the fuel cell stackwhen the estimated temperature of the MEAis less than or equal to the maximum allowable temperature of the MEA. However, when the estimated temperature of the MEAexceeds the maximum allowable temperature of the MEA, the maximum allowable output current of the fuel cell stackmay be limited to less than the maximum output current value or may be determined to be less than or equal to a predetermined current limiting value. This serves to prevent deterioration of the fuel cell stackand performance degradation due thereto.

111 111 200 230 54 240 100 100 In addition, when the estimated temperature of the MEAexceeds the maximum allowable temperature of the MEA, the fuel cell controllerserves to determine and control the rotation speed of the coolant pumpto a predetermined maximum value (i.e., maximum rotation speed) in S, and to determine and control the opening rate of the coolant control valveto a predetermined maximum value (i.e., maximum opening rate). This serves to prevent deterioration of the fuel cell stackand performance degradation due thereto by virtue of maximized cooling of the fuel cell system by maximally increasing the coolant flow rate supplied to the fuel cell stack.

111 111 100 100 100 In addition, when the estimated temperature of the MEAexceeds the maximum allowable temperature of the MEAand the maximum allowable output current of the fuel cell stackdecreases to less than the maximum output current value, the output current of the fuel cell stackis limited to less than the maximum output current value, thereby decreasing the output of the fuel cell stackand reducing the vehicle speed.

200 100 111 100 100 In this way, the fuel cell controlleris responsible for performing control to prevent deterioration and performance degradation of the fuel cell stackusing the estimated temperature data of the MEA, whereby the fuel cell stackmay be protected by appropriately maintaining the temperature of the fuel cell stack.

111 The method of thermal management of the fuel cell stack according to the present disclosure as described above may be implemented without changing the physical design of a conventional fuel cell system. In addition, according to the present disclosure, the temperature of the MEAmay be estimated using actual measurement data that may be acquired in real time, making it possible to prevent performance degradation of the fuel cell stack caused by directly measuring the temperature of the MEA.

In addition, according to the present disclosure, thermal management of the fuel cell stack may be performed based on the estimated temperature of the MEA, and thus, compared to a conventional method of thermal management of a fuel cell stack based on the coolant outlet temperature of the fuel cell stack, the maximum capacity of the coolant pump may be reduced and overdesign of the coolant pump may be prevented.

In addition, according to the present disclosure, the membrane electrode assembly may be prevented from reaching a temperature exceeding the allowable temperature thereof when the fuel cell stack deteriorates, and the fuel cell stack may be effectively protected because the output current may vary depending on the deterioration of the fuel cell stack.

7 FIG. 7 FIG. 2 1 2 1 2 1 Meanwhile,shows the MEA temperature (G) estimated by the MEA temperature estimation process according to the present disclosure compared with the MEA temperature (G) actually measured using a temperature sensor. As shown in, the error between the estimated MEA temperature (G) and the actually measured MEA temperature (G) is very small, and the average error between the estimated MEA temperature (G) and the actually measured MEA temperature (G) is 0.83%, which is within an acceptable range. Through this comparison, it can be found that the accuracy of the MEA temperature estimated according to the present disclosure is very high.

As is apparent from the foregoing, according to the present disclosure, the following effects can be obtained by performing thermal management of a fuel cell stack based on the estimated temperature of an MEA during driving of a vehicle.

First, compared to a conventional method of thermal management of a fuel cell stack based on the coolant outlet temperature of the fuel cell stack, the maximum capacity of a coolant pump can be reduced and overdesign of the coolant pump can be prevented.

Second, when the fuel cell stack deteriorates, the MEA can be prevented from reaching a temperature exceeding the maximum allowable temperature thereof.

Third, the output current can be varied to prevent deterioration of the fuel cell stack, thereby protecting the fuel cell stack from deterioration.

Fourth, the temperature of the MEA can be accurately estimated without installing a separate temperature sensor configured to measure the temperature of the MEA between unit fuel cells, and accordingly, performance degradation of the fuel cell stack caused by installing the temperature sensor between unit fuel cells can be prevented.

The effects of the present disclosure are not limited to the foregoing, and other effects of the present disclosure not mentioned herein will be clearly understood by those skilled in the art from the following description.

As the embodiments of the present disclosure have been described in detail above, the terms used in the specification and claims should not be construed as limited to ordinary or dictionary meanings thereof, and the scope of the present disclosure is limited to the aforementioned embodiments and various modifications and improvements made by those skilled in the art using the basic concept of the present disclosure defined in the claims below are also included in the scope of the present disclosure.